Work area monitor

ABSTRACT

A Work Area Monitor comprising a radar module mounted on a motorised vehicle. The Work Area Monitor provides early warning of slope failure in a work area by generating an alarm if movement detected in movement data derived from interferometrically processed radar images exceeds a threshold.

FIELD OF THE INVENTION

This invention relates to a work area monitor that employs radar todetect movement of a slope and raise an alarm if dangerous movement isdetected. The invention is particularly useful for open cut mine sitesand civil excavation sites, and potentially useful for undergroundmines.

BACKGROUND TO THE INVENTION

The use of radar to produce interferometry maps for identifying movementin a slope is known. U.S. Pat. No. 6,850,183 describes a slopemonitoring system that consists of a radar module that records radarimages of a selected slope and a video module that records visual imagesof the same slope. A data processor performs coordinate registration toalign the radar images and the visual images. Slope movement is detectedby interferometry. The invention is embodied in a product produced byGroundProbe Pty Ltd that is referred to as the SSR.

The SSR product has been used very successfully to monitor the stabilityof large slopes in open-cut mines. The SSR has detected and provided analarm prior to many hundreds of large slope failures and is widelyrecognised as an essential mine safety tool. Nonetheless, the SSR is notideal for all situations.

Mine workers are exposed to a number of major hazards including suddenor unexpected movement of ground in their immediate work area. Mineworkers are not equipped with the knowledge or tools to understandwhether a wall that they are planning to work under is and remains safe.SSR is used by mine geotechnical engineers to assess overall slopestability over an extended period, typically days or weeks, and tocritically monitor slopes that are actively moving and may becomeunsafe. The complexity of SSR allows for geotechnical engineers toassess movement types and movement rates across multiple work areas of amine from long ranges, with alarm capability to a central location.However there are specific work areas in a mine that are not adequatelycovered or alarmed by SSR. What is required is a simple short-range,fast-scanning tool that can be operated directly by a work crew and canprovide a local alarm with sufficient warning when a movement occurswithin the work area.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a Work Area Monitor(referred to as WAM).

It is another object of the invention to overcome or at least alleviateone or more of the above limitations.

Further objects will be evident from the following description.

SUMMARY OF THE INVENTION

In one form, although it need not be the only or indeed the broadestform, the invention resides in a Work Area Monitor comprising;

-   a radar module that scans a selected field of view and collects    radar images;-   a display that shows an image of the field of view;-   a processor that processes the radar images interferometrically to    extract movement data and analyse the movement data;-   an alarm that provides an audible, visible or tactile warning if the    movement data exceeds a threshold; and-   a motorised vehicle mounting the radar module and the processor.

The image of the field of view may be overlaid with the movement data onthe display. Suitably this will only be if the movement data exceeds thethreshold, although movement data below the threshold could be displayedif desired.

The Work Area Monitor may also include a camera. The camera is suitablya digital camera capable of recording sequential still images or videoimages. The camera is preferably mounted on the vehicle separate fromthe radar module. Alignment between the radar field of view and thecamera field of view is preferably set up during manufacture. The camerapreferably takes a new image during every radar scan so any movement ofthe slope measured by the radar and the location of the movement isvisually captured at the time of detection. The camera may have lowlight capability to work effectively at night.

The radar module is preferably mounted on a scanning gimbal with a scanincrement of 2° horizontally×2° vertically. This is a larger scanincrement than an SSR but the work area monitor is located much closerto the wall and the scan increment of 2.0 m×2.0 m at 60 metres range issufficient to detect rock falls, sub-bench and multi-bench failures inthe work area. The scan area may be rectangular and is selectable by theoperator to focus on the immediate work area. A typical scan time for animmediate work area is one minute or less, which is much shorter than anSSR and can provide earlier warning of rapid slope failure to the workcrew. Typical scan duration for the immediate work area is one shift, orup to 12 hours, which is much shorter than many days or weeks of scanduration for an SSR.

The processor compares successive radar images on a pixel by pixel basisto identify pixels that indicate slope movement of greater than aselected threshold. Suitably the threshold is chosen to be above theerror stack of the work area monitor. For the embodied invention, asuitable threshold of 5 mm has been chosen. The threshold may bedifferent in other embodiments but in general the inventors envisagethat the threshold will be 5 mm or less. A higher error stack over theSSR can be accommodated by the work area monitor because its scan rateis much quicker and the alarm is localised to the work area only. Theoperational range of the radar module and processor is about 30 m toabout 200 m. This range is short compared to the typical range of an SSRwhich may be 1000 m or more.

The display may display a synthetic image of the field of view generatedfrom the radar images. The synthetic image may be generated by theprocessor, or another processor specific to the application. A suitablesynthetic image is a digital terrain map. Alternatively the displaydisplays the images recorded by the camera.

The alarm is preferably an audible and visible alarm on the Work AreaMonitor platform and may further comprise a personal alarm that isremote from the Work Area Monitor but in communication with the localwork area. The personal alarm suitably receives alarms from theprocessor and generates a local alarm. The personal alarm may be carriedby a worker and suitably generates tactile and audible alarms.

The vehicle is typically a utility automobile having a cabin and trayback. The vehicle provides a motorised platform for deployment of theWork Area Monitor and the engine may provide a source of power. Thecabin may suitably be climate controlled in which case the processor,other electronics and display may be mounted in the cabin.

The vehicle may be mechanically stabilised using legs or jacks.

The work area monitor may further comprise a movement detector mountedon the vehicle that detects any movement of the vehicle that could bemisinterpreted as a slope movement. Before an alarm is raised by theprocessor the movement detector is checked to ensure the vehicle has notmoved at a time that could have caused the alarm.

The work area monitor may further comprise an anomaly detector modulethat detects any foreground activity in the field of view that couldcause a false alarm.

Suitably the work area monitor further comprises a touch screeninterface for set up and operation. Using the touch screen interface auser can select the field of view of the work area monitor and canselect a spotlight mode that selectively monitors a subset of the fieldof view.

Further features and advantages of the present invention will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist in understanding the invention and to enable a person skilledin the art to put the invention into practical effect, preferredembodiments of the invention will be described by way of example onlywith reference to the accompanying drawings, in which:

FIG. 1 shows an image of a Work Area Monitor;

FIG. 2 shows a touch screen interface of the Work Area Monitor;

FIG. 3 shows an initial set up screen;

FIG. 4 shows the screen of FIG. 3 with a work selection area;

FIG. 5 indicates how the selected radar scan and pixels are overlayed onthe camera's field of view of a slope;

FIG. 6 is a diagram showing the Work Area Monitor output derived frominterferometrically-measured displacement;

FIG. 7 demonstrates the pixels in the selected radar scan area that havealarmed;

FIG. 8 shows one form of vehicle stabilisation;

FIG. 9 shows a user screen observing a slope without movement;

FIG. 10 shows the screen of FIG. 9 but now with initial movement;

FIG. 11 shows the screen of FIG. 9 with more slope movement;

FIG. 12 shows an alert triggered by the slope movement shown;

FIG. 13 shows an alert set up and adjustment screen;

FIG. 14 shows an area masking screen;

FIG. 15 shows a playback screen;

FIG. 16 shows an example digital terrain map; and

FIG. 17 shows an alternate form of vehicle stabilisation.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention reside primarily in a Work AreaMonitor providing a visual image of a slope near a work area overlaidwith a movement map obtained from interferometrically analysed radardata. The Work Area Monitor is mounted on a motor vehicle for easymobility, comfortable user setup and reliable power source. The elementsof the Work Area Monitor have been illustrated in concise schematic formin the attached drawings, showing only those specific details that arenecessary for understanding the embodiments of the present invention,but so as not to obscure the disclosure with excessive detail that willbe readily apparent to those of ordinary skill in the art having thebenefit of the present description.

In this specification, adjectives such as first and second, left andright, and the like may be used solely to distinguish one element oraction from another element or action without necessarily requiring orimplying any actual such relationship or order. Words such as“comprises” or “includes” are intended to define a non-exclusiveinclusion, such that a process, method, article, or apparatus thatcomprises a list of elements does not include only those elements butmay include other elements not expressly listed, including elements thatare inherent to such a process, method, article, or apparatus.

Referring to FIG. 1 there is shown a first embodiment of a Work AreaMonitor 10. The Work Area Monitor 10 comprises a radar module 11 and(optionally) a camera 12 mounted on the tray of a utility motor vehicle13 or otherwise associated with the radar module 11. A processor (notvisible) is located in the cabin 14 together with the display screen 23.

The radar module 11 is a scanning dish device that consists of a dish 20mounted on a scanning gimbal 21 that has a vertical scan of −10 to +40degrees and a horizontal scan of −55 to +55 degrees. The gimbal houses a600 mm parabolic dish with an offset feed. The antenna transmits andreceives radio frequency signals in X-band with T/R gating to separatethe direct path from the wall reflection and sufficient range resolutionto separate foreground anomalies (such as mining vehicles) from the wallreflections. The mean transmitter RF power supplied to the dish is 30 mWand peak is 60 mW. A phase stable cable connects the dish to the radarmodule 11. The control for scanning the gimbal is contained in the cabin14. A computer interface connects the processor to the radar module 11,camera 12 and display screen 23.

The camera 12 has a lens that is optically optimised for a wide field ofview without distortion in the vertical plane. The camera field of viewis 120 degrees by 104 degrees. The image from the camera is displayed ona screen 23 located in the cabin 14 (as seen in FIG. 2).

It is not always practical for the vehicle to be reversed into positionso that the camera and radar point at the monitored wall over the backof the vehicle. Therefore, in one alternate embodiment, the camera isconfigured to be rotatable within a 180 degree field of view centredover the rear of the vehicle. The centre of the radar field of view isaligned with the centre of the camera field of view so that the imageviewed on the screen 23 represents what will be monitored by the radar.

In the absence of the camera the radar images are used to generate asynthetic image of the field of view. The synthetic image is able todisplay salient features in the field of view which a user is able touse for directing the Work Area Monitor. The synthetic image may be adigital terrain map, which is a 3D image comprising azimuth, elevationand range of each pixel measured by the radar. An example of a digitalterrain map is shown in the lower part of FIG. 16. The correspondingvisual image formed from a composite of photographs is shown in the toppart of FIG. 16.

The utility motor vehicle 13 may be a conventional vehicle with a dieselor petrol internal combustion engine. The engine may be left runningduring operation of the Work Area Monitor so as to provide power to theWork Area Monitor. In the preferred embodiment the Work Area Monitor isoperated from batteries that are bulk-charged by the engine of theutility motor vehicle 13. The Work Area Monitor is distinctly differentfrom other slope stability radars in the ability to operate from thetray back of a vehicle while the engine is running. This is due to theunique operating parameters of the Work Area Monitor as described below.It is also convenient to provide a mains power interface for batterytrickle-charging as is conventionally known.

The Work Area Monitor 10 is a comparatively short range device thatprovides movement monitoring of slopes at a range of about 30 metres toabout 200 metres. In operation the utility motor vehicle 13 is locatedso that the radar 11 and camera 12 point at a section of slope or wallto be monitored. An image of the slope as seen by the camera 12 isdisplayed on the screen 23. The radar dish 20 is aligned with the fieldof view of the camera 12 during manufacture so that it is known, towithin an acceptable degree of accuracy, that the centre of the field ofview of the radar corresponds with the centre of the field of view ofthe camera.

The Work Area Monitor 10 is set up by driving the vehicle 13 to alocation from which the area to be monitored can be viewed. The vehicle13 is positioned so that the radar and (optionally) the camera point atthe monitored area. The system is turned on using a single switch in thecabin 14. After a short pause an initial screen is displayed, forexample as shown in FIG. 3.

An operator selects a region of wall to monitor. In the preferredembodiment the screen 23 is a touch screen and the operator selects theregion by tracing the region on the touch screen. A grid of pixels isoverlayed on the scene to display the area that will be monitored. Thearea can be adjusted by moving the corners of the area, as shown in FIG.4. The user clicks ‘OK’ on the work area selection screen to commencemonitoring.

The grid of pixels are also used to display which pixels have moved tosound the alarm, as seen in FIG. 5 and described in greater detailbelow.

The gimbal mount 21 scans the radar dish 20 to collect reflected radarsignals at a rate to build an interferometric map of the scene in a scantime of between about 20 seconds and 2 minutes, with scan rates of oneminute or less being preferable to provide sufficient warning. Theinterferometric map is constructed in the same manner as described inU.S. Pat. No. 6,850,183. The interferometric map that is generated has adeformation precision that allows movements above the error stack of thesystem to be easily detected. FIG. 6 shows the displacement versus timeoutput of a single pixel in the interferometric map for a wallprogressing towards failure. The threshold is selected above the errorstack of the system.

In one embodiment the output of the work area monitor is a binary alarmbased on whether wall movement has been detected (‘1’) or not detected(‘0’). The Work Area Monitor operates over a short time scale of minutesand hours and alarms when there is movement in the local work arearather than providing displacement measurements of the slope over daysto weeks as is the case with the SSR. The Work Area Monitor is a shortterm monitor that can afford a higher error stack to achieve ease ofuse.

It has been found from many measurements of slope failure using the SSRthat initial movements of a few to many millimetres are apparentlyalways present as a precursor to a larger movement. Thus, as a safetydevice, the Work Area Monitor is very useful as a short term, shortrange safety device that detects movements of a few millimetres or moreand provides a warning to workers in the immediate area. Thiscompliments the operation of the SSR described in U.S. Pat. No.6,850,183, which provides longer term, high precision displacementmeasurements over a much larger wall area.

An alarm is generated by setting a threshold for detected movement. Ifthe radar processing results in a detected movement of greater than theset threshold the Work Area Monitor generates an audible and visiblealarm that warns workers that they should vacate the area. In addition,the moving area may be highlighted on the screen 23 using a colour code.FIG. 7 shows the output of the screen 23 where a wall region with redoverlay has moved. Another preferred option is to flash the pixels thatshow movement above the threshold.

The Work Area Monitor triggers an alarm based on the followingparameters:

-   Threshold: The movement in ‘mm’ of the work area to be considered as    dangerous;-   Alarming Cells: Determines the minimum size of the area to have    moved before considering the movement as dangerous;-   Looks: The number of looks over which the movement is to be    measured;-   Direction: The direction of the movement of the work area being    monitored.

If the work area contains vegetation or machinery a false alarm may begenerated. Areas that may generate a false alarm can be masked. An ‘EditMask’ mode is selected and a green mask is drawn over the problem areasas described in more detail below.

In another embodiment dimensionless alarms are calculated as describedin our earlier published international application number WO2007012112titled METHOD AND SYSTEM OF DETERMINING ALARM CONDITIONS.

In many work sites the audible and visible alarm may not be immediatelynoticed. Therefore, in one embodiment, the Work Area Monitor includes apersonal alarm that is carried by each worker. The Work Area Monitorgenerates a short range radio signal (or other suitable communicationcarrier) that triggers the personal alarm to provide a tactile andaudible alarm, typically this would be a vibration and a buzzing, towarn the carrier to leave the area.

The worker may also be provided with a display device that shows animage of the monitored wall indicating the region of movement. Moderncommunication devices, such as smart phones; are compact with relativelylarge screens. They also incorporate modems for short range and longrange wireless communication.

If an alarm has sounded or if there is a suspect rock hanging on thewall, the Work Area Monitor may be operated in a spotlight mode. Inspotlight mode a user is able to select a single pixel from the displayshown in FIG. 5. The radar then ceases scanning and focuses on theselected pixel with much faster measurement time, typically many timesper second. This allows a suspect region of the monitored slope to bemonitored more intensely.

It is possible in a mining environment that disturbances could occurthat generate a false movement alarm. Generally these disturbances fallinto two categories: non-slope movements within the field of view of theradar; movement of the Work Area Monitor. The first category ofdisturbances can be detected and ignored by detecting differential rangechanges or by comparing the short-term and long-term coherence of theradar. A movement within the radar field of view, for instance the tipof the drill rig mast 30 in the foreground of FIG. 5 and FIG. 7, willhave highly differential ranges from the slope, or will have lowshort-term coherence compared to the long-term coherence whereas amovement of the slope will have both high short-term and high long-termcoherence.

The second category of disturbances can be eliminated by using a vehiclestabilising apparatus on the front and/or rear of the vehicle, such asthose used for stabilising large recreational vehicles. One example of arear stabilisation device is shown in FIG. 8 and another example isshown in FIG. 17. The stabilisation device shown in FIG. 8 comprises apair of legs, such as 80, that are lowered to take a portion of theweight of the vehicle. A similar device is used at the front of thevehicle. The inventors have found that the stabilisation works mosteffectively if the legs take about 10% to about 20% of the weight of thevehicle.

The example shown in FIG. 17 comprises a jack (not visible) under theradar module 171. The jack lifts the radar module 171 off the tray ofthe vehicle 170. The legs 172 are extended and clamped in position sothat the radar module is supported by the ground rather than the vehicle170.

Alternatively (or as well) the Work Area Monitor can employ a movementdetection device such as tilt metres or accelerometers or other motiondetection devices. Any movement of the Work Area Monitor, such as due toa person entering the vehicle or strong wind gusts will be detected bythe movement detection device and the apparent corresponding movement ofthe slope will be ignored.

In the absence of any detected movement the screen 23 will display thework area, as shown in FIG. 9. In one embodiment, if the Work AreaMonitor 10 detects movement within the selected work area the pixelsthat have movement are highlighted, for example by rendering the pixelboundaries in yellow as shown in FIG. 10. The pixel boundaries may alsoflash to highlight the movement. If the movement continues or expandsthe manner of highlighting may adjust accordingly. FIG. 11 shows acentral few pixels that have larger or prolonged movement and aretherefore highlighted in red. Adjacent pixels with large movement thatis above the threshold may be shown in orange or red with small movementshown in yellow. Other schemes for displaying movement would also besuitable.

If the movement exceeds a threshold an alarm is triggered, such asdisplayed in FIG. 12. The alarm may be triggered, for instance, due tothe number of pixels moving, the amount of movement or the duration ofmovement. In FIG. 12 the alarm indicates that 2 cells moved more than 4mm between observations.

It will be noted that the alarm conditions can be reset or changed byselecting either option through clicking the appropriate button shown inFIG. 12. Selecting either option takes the user to an alert screen shownin FIG. 13. On the alert screen a user can adjust various alarmconditions such as the movement threshold (in millimetres), the numberof moving cells required to trigger an alarm, the number of ‘looks’ thatneed to show the movement to trigger the alarm and the direction ofmovement. The alert screen also may display an alert history in thecurrent location to assist the user to determine if the alert issignificant or possibly a false alarm. The alert level can also be set.For instance, FIG. 13 shows a configuration where a yellow alarm isnotified to local crew but a red alarm is notified to a supervisor, aswell as local crew.

The user can also elect to add a mask from the alert screen. This may berequired, for instance, if movement is apparently due to a non-slopeartefact such as the drill mast referred to earlier. Clicking the “editmask” button takes the user to the “edit alarm mask” screen shown inFIG. 14. From this screen the user can mask out certain areas usingcommon tools as shown across the top of the screen shot. Once selectedthe screen is exited by selecting “ok”.

The Work Area Monitor 10 may have facility for local recording of alimited amount of history of monitoring of an area. Operation of the WAMcan be paused to allow a user to play back the monitoring data to assistwith deciding whether detected movement presents a safety hazard. Thepause and playback screen is shown in FIG. 15. From this screen a usercan playback recent recorded slope movements. The user can also accessthe alerts screen to make changes to the alerts.

The Work Area Monitor operates in a similar manner to the SSR describedin the aforementioned United States patent but is different in a numberof significant respects. Counter-intuitively it is operated on amotorised vehicle platform that can be quickly and easily moved fromplace to place for short range monitoring. It is designed to providemuch shorter timescale monitoring with less precision but much fasterscan rates than an SSR. It provides a simple and local area alarm towarn workers of slope movement in their vicinity as opposed to thebroader pit monitoring and long-term deformation measurements of theknown SSR system. It is particularly simple to set up and operate.

Although described by reference to application in an open-cut or pitmining situation the Work Area Monitor is not limited to any specificapplication. The Work Area Monitor could also be useful for undergroundmining operations.

The above description of various embodiments of the present invention isprovided for purposes of description to one of ordinary skill in therelated art. It is not intended to be exhaustive or to limit theinvention to a single disclosed embodiment. As mentioned above, numerousalternatives and variations to the present invention will be apparent tothose skilled in the art of the above teaching. Accordingly, while somealternative embodiments have been discussed specifically, otherembodiments will be apparent or relatively easily developed by those ofordinary skill in the art. Accordingly, this invention is intended toembrace all alternatives, modifications and variations of the presentinvention that have been discussed herein, and other embodiments thatfall within the spirit and scope of the above described invention.

1. A Work Area Monitor comprising; a radar module that scans a selectedfield of view and collects radar images; a processor that processes theradar images interferometrically to extract slope movement data andanalyse the slope movement data; a motorised automobile vehicle mountingthe radar module and the processor; and a stabilisation apparatus thateliminates disturbances caused by vehicle movement.
 2. The Work AreaMonitor of claim 1, further comprising a display that shows an image ofthe field of view.
 3. The Work Area Monitor of claim 1, furthercomprising an alarm that provides an audible, visible or tactile warningif the slope movement data exceeds a threshold.
 4. The Work Area Monitorof claim 2 wherein the image is overlaid with the slope movement data.5. The Work Area Monitor of claim 2 wherein the image is overlaid withthe slope movement data if the slope movement data exceeds a threshold.6. The Work Area Monitor of claim 1 further comprising a camera.
 7. TheWork Area Monitor of claim 6 wherein the field of view of the camera isaligned with the field of view of the radar.
 8. The Work Area Monitor ofclaim 6 wherein the camera is a digital camera capable of recordingsequential still images or video images.
 9. The Work Area Monitor ofclaim 6 wherein the camera is mounted on the vehicle separate from theradar module.
 10. The Work Area Monitor of claim 1 wherein the radarmodule is mounted on a scanning gimbal.
 11. The Work Area Monitor ofclaim 10 wherein the scanning gimbal has a scan increment of 2°×°pixels.
 12. The Work Area Monitor of claim 1 having an operational rangeof about 30 m to about 200 m.
 13. The Work Area Monitor of claim 2wherein the display shows a synthetic image of the field of viewgenerated from the radar images.
 14. The Work Area Monitor of claim 13wherein the synthetic image is a digital terrain map.
 15. The Work AreaMonitor of claim 2 wherein the display shows images recorded by a cameraassociated with the radar module.
 16. The Work Area Monitor of claim 3wherein the alarm is an audible and visible alarm on the Work AreaMonitor.
 17. The Work Area Monitor of claim 3 wherein the alarm is apersonal alarm that is remote from the Work Area Monitor but incommunication with a local work area.
 18. The Work Area Monitor of claim3 wherein the alarm is a personal alarm that generates tactile andaudible alarms.
 19. The Work Area Monitor of claim 3 wherein thethreshold is chosen to be above the error stack of the work areamonitor.
 20. The Work Area Monitor of claim 3 wherein the threshold is 5mm or less.
 21. The Work Area Monitor of claim 1 wherein the vehicle isa utility automobile having a cabin and tray back.
 22. The Work AreaMonitor of claim 1 wherein the vehicle provides a motorised platform fordeployment of the Work Area Monitor and the engine provides a source ofpower.
 23. The Work Area Monitor of claim 1 wherein the stabilisationapparatus includes ground engaging legs to stabilise the vehicle andradar module.
 24. The Work Area Monitor of claim 1 wherein thestabilisation apparatus includes a jack that lifts the radar module fromthe motorised vehicle and separates the radar module from vehiclemovements.
 25. The Work Area Monitor of claim 1 wherein thestabilisation apparatus comprises a movement detector mounted on thevehicle that detects any movement of the vehicle that could bemisinterpreted as a slope movement.
 26. The Work Area Monitor of claim 1further comprising an anomaly detector module that detects anyforeground activity in the field of view that could cause a false alarm.27. A method of scanning a work area for slope failure including thesteps of: positioning and stabilising a motorised automobile vehicleadjacent a slope to be monitored for failure in a work area; directing afield of view of a radar module mounted on the motorised vehicle at theslope; selecting a region of slope to be monitored within the field ofview; collecting and interferometrically processing radar images toproduce slope movement data.
 28. The method of claim 27, furthercomprising the step of processing the slope movement data to generate analarm if the slope movement data exceeds a threshold.
 29. The method ofclaim 27 further including the step of recording an image of the fieldof view with a camera and displaying the image overlayed by arepresentation of the slope movement data.
 30. The method of claim 27further including the step of recording an image of the field of view asa radar digital terrain map and displaying the image overlayed by arepresentation of the slope movement data.
 31. The method of claim 27further including the step of checking for stability of the radar modulebefore generating an alarm.